Fix fee estimation APIs: bounded waitTime estimate, consistent with getFee (#1884) - #2450

Closes #1884

The problem

As reported in #1884, /transactions/waitTime could return absurd values (e.g. ~20 years for a 1 KB tx paying the minimal fee), while /transactions/getFee simultaneously recommended a lower fee for a 2-minute confirmation. Two causes:

1. getExpectedWaitTime and getRecommendedFee were computed from unrelated data. getRecommendedFee uses the mempool fee histogram (average fee of transactions taken from the pool, bucketed by how long they waited), while getExpectedWaitTime used position_in_pool × elapsed / takenTxns, where elapsed is the time since the start of the statistics measurement. The two could not be consistent with each other.
2. The position-based estimate was unbounded. As @pragmaxim noted in the issue, the statistics only advance when transactions are removed from the pool via ErgoMemPool#remove; when transactions get stuck, elapsed grows without limit and the estimate diverges (the "20 years" effect). Additionally, MemPoolStatistics.measurementIntervalMsec was set to 60 * 1000 (one minute) while the comment and the pruning logic (keep data up to 2*measurementIntervalMsec) clearly intend one hour.

The fix

• getExpectedWaitTime now estimates the wait time from the same fee histogram used by getRecommendedFee: the earliest wait-time bin whose average fee per KB does not exceed the queried transaction's fee per KB. This makes the two endpoints mutually consistent: a transaction paying getFee(t) gets waitTime <= t (covered by a test).
• When no suitable histogram statistics exist, the previous position-based estimate is used as a fallback, with elapsed bounded by the statistics measurement window (2 × measurementIntervalMsec), so periods with no transactions taken from the pool can no longer produce unbounded estimates.
• measurementIntervalMsec fixed to one hour (60 * 60 * 1000), matching its documentation and the histogram design (60 one-minute bins).

Testing

Four new tests in ErgoMemPoolSpec (there was no fee-estimation coverage before, addressing the testing gap mentioned by @kushti in the issue):

• getRecommendedFee returns the minimal fee when no statistics is collected;
• getRecommendedFee returns the histogram average for the first non-empty bin;
• consistency: a transaction paying the recommended fee for a t-minute confirmation gets an expected wait time <= t (and exactly the bin the recommendation came from);
• boundedness: with a year of statistics inactivity and a non-empty pool, the estimate stays within the measurement window bound.

All tests pass locally with sbt "testOnly org.ergoplatform.nodeView.mempool.ErgoMemPoolSpec".